Alzheimer' s disease (AD), a progressive neurodegenerative disorder of the elderly, is characterized by the deposition of beta-amyloid (Abeta) and neurofibrillary tangles in the hippocampus and cerebral cortex. Endoproteolytic cleavages of amyloid precursor protein (APP) by beta-and gamma-secretases result in the generation of Abeta peptides that are believed to be neurotoxic. The formation of Abeta is precluded by the endoproteolytic cleavage of APP within the Abeta sequence by a-secretase. Following the discovery of two homologous transmembrane aspartyl proteases, termed BACE1 and BACE2, studies showed that BACE1 is the beta-secretase while BACE2 cleaves at sites within the Abeta domain to limit Abeta secretion. Recent studies indicated that BACE1 might be a susceptibility factor to brain amyloidosis and an excellent therapeutic target in Alzheimer's disease. The overall goals of this proposal are to assess the role of BACE 1/BACE2 as determinants of selective vulnerability of neuron/brain to amyloidosis and to evaluate BACE1 as a high priority therapeutic target in Alzheimer's disease. To begin to test the hypothesis that the abundance of BACE1 coupled to low levels of BACE2 activity is a major determinant of selective vulnerability of neurons to Abeta amyloidosis in the brain, we will first define the levels and distributions of BACE1 and BACE2 in neurons/brain and non-neural cells/organs; the axonal transport of BACE1 and BACE2 will also be determined. The levels/activities of these proteases will be determined in different cell types and the brain and other organs of normal mice and humans, mutant APP mice, and autopsied cases of Alzheimer's disease. Second, as a direct test of the hypothesis that BACE1 is a principal determinant of brain amyloidosis, we will determine whether Abeta deposition can be respectively ameliorated or accelerated in transgenic mouse models expressing reduced or increased levels of BACE1 in brain. Finally, towards the critical evaluation of BACE1 as a therapeutic target in Alzheimer's disease, the physiological roles of BACE1 and BACE2 will be determined through the generation and analysis of BACE1-null, BACE2-null, and BACE1; BACE2 double null mice. Results from these efforts will provide important information regarding why the brain is particularly susceptible to amyloid plaque deposition and will have important implications for the design of therapeutic strategy to inhibit BACE1 in efforts to ameliorate Abeta amyloidosis in Alzheimer's disease.